1. Field of the Invention
The invention is directed to a method for setting the current through shim coils and gradient coils in nuclear magnetic resonance apparatus.
2. Description of the Prior Art
The homogeneity of the basic magnetic field is a critical factor for the imaging quality in nuclear magnetic resonance imaging apparatus. Field inhomogeneities in the imaging cause geometrical imaging distortions in the image area that are proportional to the field deviations. The field homogeneity is particularly important in the echo-planar method.
Extremely high demands are also made on the field homogeneity in nuclear magnetic resonance devices used for spectroscopy in order to achieve an adequate resolution of the spectral lines. Field inhomogeneities lead to overlapping of spectral lines.
As described in the article "Aspects of Shimming a Superconductive Whole Body MRI Magnet," G. Frese et al, Proceedings of the 9th Int. Conf. on Mag. Techn. Zurich, 9-13 September, 1985, pp. 249-251, a magnetic field can be represented with the expansion coefficients of spherical, harmonic functions. It is likewise known from this article that field deviations can be compensated by electrical shim coils. Linear field deviations, i.e., field errors of the first order, can also be compensated by charging gradient coils with an offset current, i.e., a constant current that is superimposed on a gradient pulse sequence.
Given higher demands on the field homogeneity, not only linear field deviations, but also field errors of higher orders must be compensated. For this purpose, specific shim coils that are to be charged with a suitable current are provided in addition to the gradient coils. In the imaging, the shimming, i.e., the setting of the currents via the individual shim coils as well as, if necessary, the setting of the offset current of gradient coils, is advantageously implemented before the examination of every individual patient, and typically before every measurement in spectroscopy.
The setting of the currents for the shim coils and of the offset currents for the gradient coils in order to achieve an optimum field homogeneity represents a complex problem that has hitherto often been iteratively solved. Iterative methods, however, are comparatively time-consuming, so that the dwell time of patients in the nuclear magnetic resonance tomography apparatus is lengthened. This is disadvantageous both in view of the psychological stress on the patient (particularly those given a tendency to claustrophobia) as well as in view of the possible patient throughput.
A non-iterative method for general shimming of magnets is described in the article, "Fast, Non-Iterative Shimming of Spatially Localized Signals," in Journal of Magnetic Resonance, pp. 323-334 (1992). The phase of nuclear spins is thereby identified in the direction of a plurality of projections with stimulated echo sequences. The magnetic field course in these projections can be measured on the basis of the phase curve. Thus, the coefficients of the phase curve can be identified given presentation of the magnetic field in spherical, harmonic functions. Every shim coil is allocated to a spherical, harmonic function of the n.sup.th degree and m.sup.th order. The coefficients calculated according to the above-described method are then employed as a measure for the currents to be supplied to the shim coils.